1. Field of the Invention
The present invention relates to a signal processing system in a detecting system for detecting focusing state of an optical system and, more particularly, to a system for detecting focusing state, on an object, of an optical system which is set so as to cause changes in the image formation state of the image of the object in accordance with data processing of the image formation state of the object image formed on an image pickup tube or a solid image-pickup element such as a CCD, BBD or CID.
2. Description of the Prior Art
Various conventional focus detecting systems for optical systems have been proposed. An increasing number of focus detecting systems have been proposed which utilize solid image-pickup elements such as the CCD, BBD, CID, the practicability of which has recently become notable. The characteristic feature of the image pickup tubes and solid image-pickup elements as described above resides in that extremely small photoelectric transducer elements are incorporated which produce time-serial electric signals of small picture elements of an image formed on an image formation plane. Therefore, the image pickup tube and the solid image-pickup element allow subsequent time-serial processing of the signals and are therefore suitable for processing with electric circuitry, in contrast to conventional systems which have a plurality of usual photoelectric elements on which the image of an object is formed to produce photoelectric conversion signals of the image. Since the photoelectric transducer elements constituting the solid image-pickup means, unlike the usual photoelectric transducer elements, serve to accumulate over a given period of time charge obtained by photoelectrically converting the energy of light incident thereon and to output time-serial signals, the area for each of these elements may be made very small so that the obtained time-serial signal train provides image signals of good resolution.
A focus detecting system is disclosed in U.S. Pat. No. 3,364,815, which generates an image signal representing an image of an object formed by an optical system, utilizing image-pickup means of the type described above, and which detects an image formation state of the object based on the image signal to thereby detect a focusing state of the optical system on the object. In the focus detecting system disclosed in this U.S. patent, an image signal generated by an image pickup tube is subjected to differentiation, squaring and integration to obtain a signal corresponding to an image formation state (e.g., sharpness) of the image. Based on the signal thus obtained, detection of the focusing state of the optical system is performed.
In the specification and the drawings of U.S. Ser. No. 364,815, the particular configuration of a signal processing circuit for processing the image signal is not described or illustrated. Generally, in a signal processing circuit in a focus detecting system of this type, high speed operation and simple and inexpensive circuit configuration are required. Preferably, no capacitor is included in the circuit to achieve a direct coupling, and adjustment of the offset value is made unnecessary, thus facilitating systematic integration.